How will the US economy be affected if Venezuelan oil is completely cut off due to Civil War?

Hardly at all. Venzuela’s oil production has been reduced to be barely noticeable outside the Caribbean. And right now there is a glut of oil; every time it sneaks above $55 per bbl there is a huge increase on the world market.

Citgo is the American retailer for Petroleos de Venezuela SA, the Venezuelan oil company. Citgo also gets oil from other oil wells in the USA. Impact will be mostly for Citgo but they have the other wells.